Doctor of Philosophy
Doctor of Philosophy (Ph.D.) was originally a degree granted by a university to a learned individual who had achieved the approval of his peers and who had demonstrated a long and productive career in a learned field. The appellation of "Doctor" was usually awarded only when the individual was in middle age. It indicated a life dedicated to learning, to knowledge, and to the spread of knowledge.
Nowadays, the title Ph.D. is granted to a scientist or scholar who has undertaken original research in the sciences or humanities. (In the U.S. there are often special divisions of universities, called graduate schools, which issue these degrees; in Australia, there is normally no division between undergraduate and postgraduate parts of the university.) Some ability to carry out original research has to be documented by producing a dissertation or thesis. In some countries the thesis must be given an oral defense before a committee. The degree is a prerequisite for permanent employment as a university lecturer or as a researcher in some sciences.
In several countries (U.S., Australia) most postgraduate students doing research in this level complete a Ph.D. degree, no matter what subject area they are doing research in. In other countries, these degrees are distinguished by subject area ("Doctor of Natural Sciences", "Doctor of Social Sciences"). (However, even in the U.S., there are sometimes separate degrees, e.g. Th.D. for theology). In some countries, the corresponding degree is simply called "Doctor".
PhDs are distinguishable from higher doctorates (such as DLitt or DSc), which are issued by a committee on the basis of a long record of research and publication. Sometimes PhDs are issued honoris causa, but more commonly higher doctorates are used for this purpose.